Hi,
I believe that my first Japanese Music experience was probably through anime; I saw Lensman when I was 8 years old which had a nice soundtrack. However, as for music from popular japanese bands, I think I can safely say that it's mostly thanks to my interest in Japanese music games. Beatmania, Pop'n Music, Dance Dance Revolution... I love most of their soundtracks.
My fondness of Japanese popular music is fairly limited though. I find I'm not overly fond of bands such as the visual-kei bands you often see... but I do give most stuff a listen when I come across a new song or band to see if a new favourite is there.
Haven't had much luck... I seem to like one off songs from a lot of different artists.